This invention relates to energy storage systems and more particularly to a battery management system for improving the performance of batteries.
The battery industry has seen increased demand for battery management technology, primarily due to the consumers"" ever-increasing requirements for the convenience of battery-powered portable equipment such as cellular phones and laptop computers. Additionally, the battery industry is seeing a movement toward an increased emphasis on electric motor-driven tools and zero emission vehicles with the primary power source for these new generation vehicles being batteries. This movement is due to rapidly increasing government regulations and consumer concerns about air and noise pollution. Another area which requires high efficiency batteries is energy storage applications such as load-levelling, emergency/standby power and power quality systems for sensitive electronic components.
As a result of the increasing demand of battery-powered equipment, the battery industry is under competitive pressure to produce an ideal cell. An ideal cell is a cell that weighs almost nothing, takes up no space, provides excellent cycle life and has ideal charge/discharge performance and does not itself produce an environmental hazard at the end of its life. The most popular technology utilised by the battery industry is the lead-acid battery, which is being challenged to meet higher energy density, smaller size, better performance levels, longer cycle life and guaranteed recyclability.
Several manufacturers are researching exotic batteries, including nickel-metal-hydride, lithium-ion and the like but generally these types of batteries are too expensive to make their use economically viable at this stage, particularly for one of the fastest growing markets on earth, two/three wheeled passenger vehicles. It is well recognised that battery performance, even that of the existing lead-acid battery, can be improved through proper management of the operating conditions of the battery.
There are several aspects of battery management that are not being adequately addressed at this stage, these include;
(i) protection against overcharge during recharge or regeneration operations,
(ii) protection against over discharge during high power draw or long duration operations,
(iii) minimisation of the negative effects of internal resistance of the battery, and
(iv) the ability to monitor, control and protect individual cells of a battery system,
Lead-acid battery chargers typically have two tasks to accomplish. The first is to restore capacity, often as quickly as possible, and the second is to maintain capacity by compensating for self-discharge. In both instances, optimal operation requires accurate sensing of battery voltage and temperature. When a typical lead-acid cell is charged, the lead sulphate is converted to lead and lead dioxide on the battery""s negative and positive plates respectively. Over-charge reactions begin when the majority of the lead sulphate has been converted, typically resulting in the formation of hydrogen and or oxygen gas due to the breakdown of the electrolyte, this is typically referred to as xe2x80x9cgassingxe2x80x9d. In vented or valve regulated batteries this leads to a loss of electrolyte and dehydration of the electrolyte will occur, thereby affecting the cycle life of the battery.
The onset of over-charge can be detected by monitoring battery voltage. Over-charge reactions are indicated by a sharp rise in the cell""s voltage. The point at which over-charge reactions begin is dependent on the charge rate, and as charge rate is increased, the percent of return capacity at the onset of over-charge diminishes ie. The energy used in overcharging can not be recovered from the battery. Controlled overcharging is typically employed to return full capacity as soon as possible and to attempt to bring an unbalanced battery back into balance, however, at the price of reducing cycle life.
Although several methods are used to recharge batteries, all methods consider the group of individual cells as one unit and do not actually monitor each individual cell of a particular battery, which is vital to provide a true balance within the group of cells. A typical 12 volt battery is comprised of 6 individual 2 volt cells connected in series, within a casing with a main terminal for the primary connections. Typically, battery cells do not perform identically and during charge and discharge functions the cells eventually degenerate to an xe2x80x9cout of balancexe2x80x9d state.
Two critical aspects of cell life are the upper and lower voltage levels. If a 2 volt cell of a lead-acid battery exceeds approximately 2.6 volts during recharge or regeneration functions it will gas which causes electrolyte dehydration and affects cell life. If the cell voltage drops below approximately 1.6 volts during discharge functions then permanent damage to the surface of the plates can occur. With most conventional charging systems, the battery charger is only connected to the first and last terminal of the series of cells and therefore cannot accurately monitor and protect the individual cells from damage. Typically, a charger only sees and reacts to the accumulated voltage with the result that the good cells are actually over-charged to bring one weak cell up to a high enough voltage for the accumulated total to meet the charger""s predetermined requirements. This over-charging, dehydrates the electrolyte and starves the good cells, seriously affecting the cycle life of not only the cells, but the total battery.
The internal resistance of a battery is another factor which greatly affects the charge and discharge capabilities of the battery system. Batteries suffer a number of problems, which result in a loss of performance, however, one of the main limitations is overcoming the internal resistance. Every battery system has an internal resistance but the aim is to minimise the internal resistance and at the same time store the maximum amount of energy per unit weight. When a load is applied to a battery system the required current flows and a drop in battery voltage results due to the internal resistance of the battery. The lower the resistance the lower the voltage drop of the battery. This is due to the total internal resistance of the battery, which comprises of the physical resistance of the components and the resistance due to polarisation such as activation and concentration polarisation.
A significant contribution to the total internal resistance of any battery system is polarisation in its simplistic form, concentration polarisation involves a build up of reactants or products at an electrode""s surface, which limit the diffusion of reactants to the electrodes and products away from the electrodes. The higher the current the higher the polarisation losses that can be experienced by a battery system. Therefore, the highest current that can be extracted from battery systems is limited by the degree of polarisation within a battery system. However, if the polarisation losses can be controlled, much higher currents at minimal voltage losses should be obtainable from most battery systems.
It is, therefore, an object of the present invention to provide a power control device for providing a predetermined power output from a battery which significantly reduces the internal resistance losses experienced with most types of batteries.
According to one aspect of the invention there is provided a power control system for providing a predetermined power output from a battery system comprising:
(i) output means for delivering power from the system to a load,
(ii) control means adapted to be connected to the battery system to sense pre-selected operating parameters of the battery system in a first mode of operation, and in a second first mode of operation to provide power from the battery system to the output means,
(iii) first capacitor means adapted to store a predetermined quantity of power when said control means is in the first mode of operation, said first capacitor means connected between the control means and the battery system and adapted to supply its stored power to the battery system in response to a command signal from the control means when the control means is in the second mode of operation,
(iv) second capacitor means adapted to store a predetermined quantity of power when said control means is in a fist mode of operation, said second capacitor means connected between the control means and the output means and adapted to supply its stored power to the output means in response to a command signal from the control means when the control means is in the second mode of operation.
Preferably, the first and second capacitor means are adapted to store a small percentage of the power being transferred out of the battery.
In one embodiment of the invention, the control means provides the command signals to the first capacitor means and the second capacitor means at a predetermined time interval after the commencement of supply of power from the power control system.
In another embodiment of the invention, the control means is adapted to sense the polarisation level in the battery and the control signals to the first capacitor means and the second capacitor means are initiated when the polarisation level in the battery drops below the predetermined limit.
The stored power in the first capacitor means induces a reverse charge or pulse to excite the electrodes within the battery system at a rate that is proportional to the internal resistance of the battery system as sensed by the control means. The excitation of the surfaces of the electrodes permits greater current flow into and out of the battery and thereby permits greater current draw, faster re-charge and longer cycle life for the battery system.
The control system can be adapted to sense the pre-selected operating parameters of the battery system as a whole or the individual cells comprising the battery system.
The power control device may be adapted to monitor automatically the current flow, temperature, internal resistance and operating performance of the battery. The power control device may be further adapted to monitor each individual cell of the battery system during both the charge and discharge cycles.
According to another aspect of the invention, the power control system as described above may be used to provide a predetermined power input to a battery system from a battery charger.
According to another aspect of the invention there is provided a management system for a battery having at least one cell that has at least a pair of electrodes and which is susceptible to polarisation, said battery management system comprising:
(i) means for monitoring a predetermined parameter of the or each cell that is indicative of the level of polarisation,
(ii) means for storing a predetermined amount of the power being transferred into or out of the battery, and
(iii) means for inducing a reverse charge or pulse to the electrodes according to the level of polarisation so as to reduce the polarisation loss.